2018.02.20
System draws power from daily temperature swings
Technology developed at MIT can harness temperature fluctuations of many kinds to produce electricity.
Thermoelectric devices, which can generate power when one side of the device is a different temperature from the other, have been the subject of much research in recent years. Now, a team at MIT has come up with a novel way to convert temperature fluctuations into electrical power. Instead of requiring two different temperature inputs at the same time, the new system takes advantage of the swings in ambient temperature that occur during the day-night cycle.
The new system, called a thermal resonator, could enable continuous, years-long operation of remote sensing systems, for example, without requiring other power sources or batteries, the researchers say.
The findings are being reported in the journal Nature Communications, in a paper by graduate student Anton Cottrill, Carbon P. Dubbs Professor of Chemical Engineering Michael Strano, and seven others in MIT’s Department of Chemical Engineering.
“We basically invented this concept out of whole cloth,” Strano says. “We’ve built the first thermal resonator. It’s something that can sit on a desk and generate energy out of what seems like nothing. We are surrounded by temperature fluctuations of all different frequencies all of the time. These are an untapped source of energy.”
While the power levels generated by the new system so far are modest, the advantage of the thermal resonator is that it does not need direct sunlight; it generates energy from ambient temperature changes, even in the shade. That means it is unaffected by short-term changes in cloud cover, wind conditions, or other environmental conditions, and can be located anywhere that’s convenient — even underneath a solar panel, in perpetual shadow, where it could even allow the solar panel to be more efficient by drawing away waste heat, the researchers say.
The thermal resonator was shown to outperform an identically sized, commercial pyroelectric material — an established method for converting temperature fluctuations to electricity — by factor of more than three in terms of power per area, according to Cottrill.
The researchers realized that to produce power from temperature cycles, they needed a material that is optimized for a little-recognized characteristic called thermal effusivity — a property that describes how readily the material can draw heat from its surroundings or release it. Thermal effusivity combines the properties of thermal conduction (how rapidly heat can propagate through a material) and thermal capacity (how much heat can be stored in a given volume of material). In most materials, if one of these properties is high, the other tends to be low. Ceramics, for example, have high thermal capacity but low conduction.
To get around this, the team created a carefully tailored combination of materials. The basic structure is a metal foam, made of copper or nickel, which is then coated with a layer of graphene to provide even greater thermal conductivity. Then, the foam is infused with a kind of wax called octadecane, a phase-change material, which changes between solid and liquid within a particular range of temperatures chosen for a given application.
A sample of the material made to test the concept showed that, simply in response to a 10-degree-Celsius temperature difference between night and day, the tiny sample of material produced 350 millivolts of potential and 1.3 milliwatts of power — enough to power simple, small environmental sensors or communications systems.
http://science.gov.az/news/open/7430
